Some medical treatments rely on implanting a medical material, such as a time-released drug or a radiation source, at a target site within a patient to direct localized action. For example, brachytherapy is a form of internal radiation therapy in which radioactive materials are introduced near or within a tumor of a cancer patient. Such radioactive materials may provide a high dose rate (HDR) treatment during transient implantation, and then may be removed. Alternatively, low dose-rate (LDR) materials may be implanted more permanently in the cancer patient and allowed to decay radioactively over a longer time period.
LDR brachytherapy is used commonly for treating prostrate cancer. In such LDR treatment, radioactive “seeds” act as radiation sources implanted at predefined regions within (or near) a prostate tumor, directing a sustained, localized dose of radiation to the tumor, with reduced radiation exposure to surrounding healthy tissue.
Cannula/stylet assemblies are utilized to deliver the radioactive seeds to tumors during LDR brachytherapy. A cannula (or needle) having a central bore receives the seeds in the bore, and a distal end of the cannula is inserted into a tumor. The cannula also receives a stylet in the central bore at a proximal end of the cannula. The seeds are implanted in the tumor by retracting the proximal end of the cannula over the stylet. This process ejects the seeds from the distal end of the cannula along a path in the tumor defined by the distal end as it is pull through the tumor. Alternatively, the seeds may be placed within or near the tumor using other techniques, for example, during surgery.
The seeds may be positioned more precisely and stably in the tumor by arraying the seeds beforehand using positioning elements. One such positioning element, termed a carrier, may be disposed around the seeds, to enclose or encapsulate a set of the seeds. The carrier may prevent seeds from migrating away from their sites of delivery within a tumor, thus reducing undesired exposure of adjacent healthy tissue. Alternatively, or in addition, other positioning elements, termed spacers, may be disposed between seeds to define the spacing between adjacent seeds or from the end of a carrier. Accordingly, spacers may be useful to distribute a radiation dose more uniformly and precisely within the tumor.
Since carriers and spacers are not removed manually after delivery to tissue, they may be configured beneficially to be bioabsorbable. In particular, their rate of bioabsorption may be a least several-fold longer than the effective lifetime of the radioactive seeds, so that the carriers and spacers continue to position the seeds until the seeds are no longer providing a therapeutic dose of radiation. Bioabsorbable materials used to produce carriers and spacer may be natural or synthetic.
Natural materials, such as catgut, have been used to form bioabsorbable carriers. However, these materials may be inadequate for a number of reasons. For example, such natural materials may be difficult to adapt to manufacturing processes, resulting in carriers with non-uniform shapes and/or inconsistent diameters. In addition, many natural materials are fibrous and thus the carriers may fray. As a result, these carriers may not travel smoothly within the cannula during loading and ejection, and thus may compromise seed implantation and subsequent tumor irradiation. Furthermore, carriers formed of natural materials may be difficult to sterilize and may carry impurities with unwanted biological activities.
Synthetic materials also have been used to form carriers, as an assembly of fibers (see FIG. 1). The assembly forms a tube 20 from a plurality of thin, solid fibers 22 that are braided or woven in a tubular configuration around a removable core 24. Tube 20 generally is flexible and expandable as manufactured, but, with heating, the tube can be rigidified. However, such multi-fiber carriers suffer from some of the same problems associated with carriers formed of natural materials. For example, they may tend to stick within a cannula because they have a varying diameter, lack a smooth exterior surface, and/or have a tendency to fray.